Re: [PATCH v10 4/11] LSM: syscalls for current process attributes

[Paul Moore <paul@paul-moore.com>]

On Apr 28, 2023 Casey Schaufler <casey@schaufler-ca.com> wrote:
> 
> Create a system call lsm_get_self_attr() to provide the security
> module maintained attributes of the current process.
> Create a system call lsm_set_self_attr() to set a security
> module maintained attribute of the current process.
> Historically these attributes have been exposed to user space via
> entries in procfs under /proc/self/attr.
> 
> The attribute value is provided in a lsm_ctx structure. The structure
> identifies the size of the attribute, and the attribute value. The format
> of the attribute value is defined by the security module. A flags field
> is included for LSM specific information. It is currently unused and must
> be 0. The total size of the data, including the lsm_ctx structure and any
> padding, is maintained as well.
> 
> struct lsm_ctx {
>         __u64 id;
>         __u64 flags;
>         __u64 len;
>         __u64 ctx_len;
>         __u8 ctx[];
> };
> 
> Two new LSM hooks are used to interface with the LSMs.
> security_getselfattr() collects the lsm_ctx values from the
> LSMs that support the hook, accounting for space requirements.
> security_setselfattr() identifies which LSM the attribute is
> intended for and passes it along.
> 
> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
>  Documentation/userspace-api/lsm.rst |  15 ++++
>  include/linux/lsm_hook_defs.h       |   4 +
>  include/linux/lsm_hooks.h           |   9 +++
>  include/linux/security.h            |  19 +++++
>  include/linux/syscalls.h            |   5 ++
>  include/uapi/linux/lsm.h            |  36 +++++++++
>  kernel/sys_ni.c                     |   4 +
>  security/Makefile                   |   1 +
>  security/lsm_syscalls.c             |  55 ++++++++++++++
>  security/security.c                 | 112 ++++++++++++++++++++++++++++
>  10 files changed, 260 insertions(+)
>  create mode 100644 security/lsm_syscalls.c

...

> diff --git a/security/security.c b/security/security.c
> index 5a48b1b539e5..94b78bfd06b9 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2176,6 +2176,118 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
>  }
>  EXPORT_SYMBOL(security_d_instantiate);
>  
> +/**
> + * security_getselfattr - Read an LSM attribute of the current process.
> + * @attr: which attribute to return
> + * @ctx: the user-space destination for the information, or NULL
> + * @size: the size of space available to receive the data
> + * @flags: special handling options. LSM_FLAG_SINGLE indicates that only
> + * attributes associated with the LSM identified in the passed @ctx be
> + * reported
> + *
> + * Returns the number of attributes found on success, negative value
> + * on error. @size is reset to the total size of the data.
> + * If @size is insufficient to contain the data -E2BIG is returned.
> + */
> +int security_getselfattr(unsigned int attr, struct lsm_ctx __user *ctx,
> +			 size_t __user *size, u32 flags)
> +{
> +	struct security_hook_list *hp;
> +	struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
> +	u8 __user *base = (u8 __user *)ctx;
> +	size_t total = 0;
> +	size_t entrysize;
> +	size_t left;
> +	bool toobig = false;
> +	int count = 0;
> +	int rc;
> +
> +	if (attr == 0)
> +		return -EINVAL;
> +	if (size == NULL)
> +		return -EINVAL;
> +	if (get_user(left, size))
> +		return -EFAULT;
> +
> +	if ((flags & LSM_FLAG_SINGLE) == LSM_FLAG_SINGLE) {
> +		if (!ctx)
> +			return -EINVAL;
> +		if (copy_struct_from_user(&lctx, sizeof(lctx), ctx, left))
> +			return -EFAULT;
> +		if (lctx.id == LSM_ID_UNDEF)
> +			return -EINVAL;
> +	} else if (flags) {
> +		return -EINVAL;
> +	}
> +
> +	hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
> +		if (lctx.id != LSM_ID_UNDEF && lctx.id != hp->lsmid->id)
> +			continue;

I think we're missing a copy_struct_from_user() call somewhere; how
does @lctx get populated in the non-LSM_FLAG_SINGLE case?  How does
it move to the next entry in the buffer?

Am I missing something obvious?  Was this code tested?

> +		entrysize = left;
> +		if (base)
> +			ctx = (struct lsm_ctx __user *)(base + total);
> +		rc = hp->hook.getselfattr(attr, ctx, &entrysize, flags);
> +		if (rc == -EOPNOTSUPP) {
> +			rc = 0;
> +			continue;
> +		}
> +		if (rc == -E2BIG) {
> +			toobig = true;
> +			left = 0;
> +			continue;
> +		}
> +		if (rc < 0)
> +			return rc;
> +
> +		left -= entrysize;
> +		total += entrysize;
> +		count += rc;
> +	}
> +	if (put_user(total, size))
> +		return -EFAULT;
> +	if (toobig)
> +		return -E2BIG;
> +	if (count == 0)
> +		return LSM_RET_DEFAULT(getselfattr);
> +	return count;
> +}
> +
> +/**
> + * security_setselfattr - Set an LSM attribute on the current process.
> + * @attr: which attribute to set
> + * @ctx: the user-space source for the information
> + * @size: the size of the data
> + * @flags: reserved for future use, must be 0
> + *
> + * Set an LSM attribute for the current process. The LSM, attribute
> + * and new value are included in @ctx.
> + *
> + * Returns 0 on success, -EINVAL if the input is inconsistent, -EFAULT
> + * if the user buffer is inaccessible or an LSM specific failure.
> + */
> +int security_setselfattr(unsigned int attr, struct lsm_ctx __user *ctx,
> +			 size_t size, u32 flags)
> +{
> +	struct security_hook_list *hp;
> +	struct lsm_ctx lctx;
> +
> +	if (flags)
> +		return -EINVAL;

Once again, I don't see a reasonable way to support setting an
attribute across multiple LSMs, but for the sake of consistency
across both the getselfattr and setselfattr syscalls, what do
people think about *requiring* the LSM_FLAG_SINGLE flag here in
the setselfattr syscalls since it can only operate on a single
LSM at a time?  A zero flag value would return -EINVAL.

> +	if (size < sizeof(*ctx))
> +		return -EINVAL;
> +	if (copy_from_user(&lctx, ctx, sizeof(*ctx)))
> +		return -EFAULT;
> +	if (size < lctx.len || size < lctx.ctx_len + sizeof(ctx) ||
> +	    lctx.len < lctx.ctx_len + sizeof(ctx))
> +		return -EINVAL;
> +
> +	hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
> +		if ((hp->lsmid->id) == lctx.id)
> +			return hp->hook.setselfattr(attr, ctx, size, flags);
> +
> +	return LSM_RET_DEFAULT(setselfattr);
> +}
> +
>  int security_getprocattr(struct task_struct *p, int lsmid, const char *name,
>  			 char **value)
>  {
> -- 
> 2.39.2

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